Process for producing liquefied natural gas

Description

FIELD OF THE INVENTION

The present invention relates to a method for liquefying methane-rich gas containing higher hydrocarbons.

BACKGROUND

In the production of liquid methane-rich gas, such as liquid natural gas (LNG) it is generally desired to reduce its content of C5+ hydrocarbons to around 0.1 mol % and of aromatic compounds to below 1 mol ppm to avoid such materials solidifying in the heat exchangers of the liquefaction process. The content of such higher hydrocarbons is normally reduced by means such as cooling the feed gas and removing the condensed liquid, or by washing the feed gas with a suitable hydrocarbon liquid in a so-called “scrub column”, or by the use of a solid adsorbent.

However, when the pressure of the feed gas is much higher than 50 bar, the above-mentioned techniques may be insufficient to achieve the desired levels of residual higher hydrocarbons. In such instances provision can be made for the pressure of the feed gas to be reduced significantly, typically in a work expander, its heavy hydrocarbon content then reduced by condensation or scrubbing, and the depleted feed gas recompressed to near its original pressure upstream for the liquefaction step.

SUMMARY OF THE INVENTION

According to the invention there is provided:

a process for liquefying natural gas or other methane-rich gases comprising

• • providing a stream of feed methane-rich gas at a pressure of from 40 to 120 bar and containing higher hydrocarbons;
  • providing a stream of methane-rich recycle gas at a pressure of from 40 to 120 bar;
  • mixing the feed gas with a first part of the recycle gas;
  • passing the resulting mixture to a gas expander, the expander outlet having a pressure of between 3 bar and 50 bar, so as to form a mixture of vapour and a condensed liquid containing higher hydrocarbons (C5+ hydrocarbons and/or aromatic compounds);
  • separating the expander outlet stream into a liquid stream and a vapour stream;
  • reheating and compressing said vapour stream to a pressure of from 40 to 120 bar to form a first constituent of the above-said recycle gas;
  • cooling a second part of the said recycle gas to a temperature higher than the outlet temperature of the said gas expander;
  • passing said cooled second part of the recycle gas into a liquefaction unit to form liquefied methane and a second vapour stream;
  • reheating and compressing said second vapour stream to a pressure of from 40 to 120 bar to form a second constituent of the above-said recycle gas.

The invention comprises an adaptation of methane expander based LNG processes, and particularly of the dual methane expander process described in WO 2012/172281, whereby the feed gas is supplied to the said expander and the desired quantity of condensed heavy hydrocarbons is separated from the expander outlet stream.

The invention is applicable particularly to floating LNG production, due to the potential for reducing weight and deck area, and to small scale land-based LNG production from higher pressure natural gases.

The pressure of the feed methane-rich gas is preferably from 50 to 100 bar in which case the recycle gas is preferably also pressurised to 50 to 100 bar. The outlet pressure of the gas expander is preferably from 5 to 30 bar.

Optionally, the mixture of feed gas and part of the recycle gas is cooled in a heat exchanger before admission to the gas expander. Optionally, the outlet stream from the gas expander may be heated or cooled to vary the quantity of higher hydrocarbons in the liquid.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention will be further described with reference to the accompanying drawings in which FIG. 1 represents a flow diagram illustrating a process in accordance with the invention.

The exact flow sheet will depend upon the feed gas specification, but will generally contain these basic elements. Where pressures are stated anywhere in this application as “bar”, these are bar absolute.

The feed natural gas (1) is passed through a pretreatment stage A in which components such as acid gases, water vapour and mercury may be removed to produce a pre-treated gas (2).

The pre-treated gas is mixed with a first part (4) of a recycle gas (3), described below, comprising typically 30% to 60% of the total recycle gas flow on a molar basis. In the resulting mixture the ratio of the molar flow of the recycle gas to the molar flow of feed gas is typically in the range of 0.5 to 2. The resulting mixture (5), after optionally cooling (6) in cooler B, flows to a gas expander machine C at a pressure of between 40 and 120 bar, more typically between 50 and 100 bar.

The outlet from expander C, stream (7) has a pressure of between 3 bar and 50 bar, and more typically between 5 bar and 30 bar may contain a condensate comprising C5+ and/or aromatic compounds. Stream (7) may optionally be further cooled in cooler D (stream 8) so as to increase the amount of condensate formed.

The partially condensed stream (7 or 8) is separated into a liquid (9) and a vapour (10) in separator E. Typically stream 9 contains lighter hydrocarbons in addition to the aforesaid condensed heavy hydrocarbons. This stream will typically be removed from the process for use as fuel, or may be separated into lighter and heavier fractions, with the lighter fraction optionally recycled. In a further option Separator E may form the upper part of a demethaniser column. All these options for separation and subsequent processing of Stream 9 do not form part of the invention.

The vapour (10) from separator E is typically reheated in a first cold passage of heat exchanger F and the stream (11) compressed in compressor G to a pressure of 40 to 120 bar (stream 12) and then cooled in cooler H to form a first constituent of the aforementioned recycle gas (3).

A second part (Stream 13) of the recycle gas (3) is cooled (14) in a hot passage of heat exchanger F and is then passed into a liquefaction unit N shown in dotted outline. The products of the liquefaction unit are liquefied methane (LNG) and a vapour stream (23). In the liquefaction unit the stream (14) is divided. A first part (15), which typically comprises 25% to 35% of Stream 14, is further cooled in a hot passage of heat exchanger I, to form a methane-rich condensate or dense phase (16), which may be depressurised in a valve or turbine J (Stream 17) to produce LNG product.

Whilst the example is based on a liquefaction unit N generally in accordance with WO 2012/172281, other types of liquefaction units could be substituted. In particular, a liquefaction unit which achieved complete liquefaction of the said second part of the recycle gas (14) so that the second vapour stream (23) is zero could be employed.

To provide the most part of the necessary cooling in heat exchanger I, a second part (18) is expanded in a second gas expander K. Any liquid in the expander outlet (19) is separated (20) in separator L and depressurised through valve or turbine M to produce additional LNG product (21).

The vapour from separator L (22) is reheated in a cold passage of heat exchanger I and stream (23) reheated in a second cold passage of heat exchanger F. Stream (24) is then compressed in compressor G to a pressure of from 40 to 120 bar to form a second constituent of the aforementioned recycle gas (stream 3).

According to the invention the pressure of stream (24) may be higher or lower than the pressure of stream (11).

An example of the removal of heavy hydrocarbon and aromatic material is provided in Table 1 (page 8). The benzene concentration of the feed (2) of 1000 mol ppm is reduced to 1 mol ppm in stream (10). Stream (10) has a composition close to the composition of the LNG product.